This invention relates to cross-linked polymers derived from polyaniline, to gels comprising the cross-linked polymers, and to articles fabricated therefrom.
A great deal of research effort has been directed to polyaniline, due in large part to its excellent stability and the relatively high levels of electrical conductivity of certain of its chemical and structural forms. For example, one form of polyaniline known as "emeraldine base" or "polyaniline base"--depicted in structure (1), wherein Y=0.5--can be protonated by dilute aqueous protonic acid such as hydrochloric acid to produce the corresponding salt--depicted in its bipolaronic form in structure (2), wherein Y=0.5 and A=Cl--which exhibits conductivities of about 1-5 Siemans per centimeter (S/cm) when in the form of a compressed powder pellet. ##STR1##
The protonated or doped form of emeraldine base exhibits the highest conductivity of any unsubstituted polyaniline. It will be understood that the term "unsubstituted polyaniline" refers to compounds having structures such as (1) and (2), wherein hydrogen atoms are attached to the carbon/nitrogen backbone. Unsubstituted polyanilines are to be contrasted with substituted polyanilines--also known as polyaniline derivatives--wherein one or more of the hydrogen atoms are replaced by alkyl, alkoxyl, or other types of chemical functional groups It will understood that the term "polyaniline" as used hereinafter refers to unsubstituted compounds and, unless otherwise noted, to oxidation states wherein Y is approximately 0.5.
For many applications, it is necessary that conducting polyaniline polymers be processed into shaped articles such as fibers, films, and composites thereof. Unfortunately, however, it is typically quite difficult to fabricate shaped articles from conductive polyaniline, even though these polymers can be synthesized with relative ease. The difficulties which attend the processing of conductive polyanilines are largely attributable to the infusibility of the polymers in both doped and undoped forms. Also, there appear to be few solvents which will form polymer solutions from which the polymers can be recovered substantially unchanged. For example, the only such solvent showing useful solubility for emeraldine base is 1-methyl-2-pyrrolidinone (NMP). Emeraldine base can also be dissolved in an 80 weight percent solution of acetic acid, from which films of doped emeraldine acetate can be recovered upon solvent evaporation. Emeraldine base can also be dissolved in concentrated sulfuric acid, from which emeraldine sulfate can be recovered by diluting the solution with water.
Films and fibers of doped and undoped polyaniline can be obtained from the solvents noted above, but there presently is no convenient way to obtain shaped articles of either the pure doped or undoped polymer. Even if objects could be obtained from the base forms of the polyanilines, dopant likely would diffuse into such objects very slowly. As a result, costly and time-consuming processing steps would be necessary to produce articles comprising conductive polyaniline. Considerable attention has therefore been devoted to the development of polyaniline derivatives having improved processing properties, particularly solubility.
A gel, as will be appreciated by those skilled in the art, is a form of matter which is intermediate between a solid and a liquid. Gels typically comprise a cross-linked polymer network which is swellable in a selected liquid medium. Given the wide variety of polymers and liquids known in the art, an exceedingly wide variety of gels may theoretically be prepared.
The properties of a gel depend strongly on the chemical composition and interaction of its constituent polymer and liquid. For example, it has been found that drastic volume changes for many gels can be effected by changing the composition of the liquid and also through temperature variation, irradiation, and the application of electric fields. Those skilled in the art will recognize that the expansion of a gel in response to a liquid is more commonly known as swelling.
While the behavior of gels has generated considerable research interest, few of the gels studied to date have been prepared from electrically conductive polymers. Yoshino, et al. (J. Phys. : Condens. matter, 1990, 2, 2857, Solid State Communications, 1989, 70, 609, and Japanese Journal of Applied Physics, 1989, 28, 682) have disclosed gels which contain conductive poly(3-alkylthiophene) having structure (3). ##STR2## Yoshino, et al. indicate that the volume, shape, and color of poly(3-alkylthiophene) gels changes drastically with solvent composition and temperature, and that drastic shrinkage is observed upon doping the polymer with iodine. This shrinkage was attributed to cross-linking between polymer chains. Yoshino, et al., hypothesized that conductive polymer gels may be useful as molecular actuators and other applications where shape memory is desired.